1. Field of the Invention
This invention relates to the removal of arsenic compounds from light hydrocarbonaceous streams which contain arsenic and mercaptan sulfur compounds. The feedstock stream can be a petroleum derived naphtha or it can be a synthetic naphtha derived from shale oil, coal liquefaction, tar sands, etc. The naphtha boiling range can be broadly 90.degree.-450.degree. F., more usually 100.degree.-400.degree. F., or as used in the following tests 140.degree.-380.degree. F.
The feedstock can also be liquefied petroleum gas (LPG), nominally liquefied propane. Still another suitable feedstock can be light liquid hydrocarbons in the C.sub.3 -C.sub.5 range. In general, the feedstock can be any hydrocarbonaceous liquid containing arsenic and mercaptan sulfur compounds wherein the mercaptans are susceptible to catalytic oxidation to form organic disulfides.
Various crude oils, such as Taching (China) crude, West Texas crudes, certain Russian crudes, etc., have arsenic compounds as contaminants along with the more normal impurities which contain the elements sulfur, nitrogen and oxygen. When a naphtha cut is distilled from crude containing arsenic, the naphtha also contains arsenic compounds. The naphtha will also contain organic sulfur compounds such as mercaptans, organic sulfides and organic disulfides.
There are many well known and practiced methods for eliminating sulfur compounds from naphthas. However, there are no known methods for removing arsenic compounds in the presence of sulfur compounds from naphtha. Feed naphthas to ethylene plants including furnaces and downstream catalytic reactors should be substantially free of trace arsenic (20-2000 parts per billion) (PPB) and yet contain organic sulfur compounds to be ideal ethylene feed stocks. The reason there is no previously known method for removing arsenic without removing sulfur is that arsenic removal catalysts are also active for sulfur removal. The sulfur is usually present in a much higher concentration level than is the arsenic and so it deprives the catalyst of arsenic removal capacity.
Downstream arsenic as arsine passes through purification units and poisons noble metal catalysts. Arsenic is a serious poison in these units even at 50 PPB levels. Also, arsenic deposits on high temperature naphtha cracker tube surfaces to cause coke build-up, "hot" tubes, tube failure, reduced production and reduced product selectivity.
On the other hand, organic sulfur is a desirable impurity in feed naphthas to ethylene furnaces (steam-naphtha cracking). It passivates nickel-cobalt-containing metal alloy tubes at temperatures in the range 1600.degree.-1800.degree. F. so that destructive hydrogenolysis and/or undesired cracking reactions, including demethanation, do not take place. The organic sulfur is thermally converted in the tubes to H.sub.2 S which sulfides the metal surface, thereby passivating the surface and making it inert to the reaction environment. The sulfur must be continually replaced at the tube surface and, therefore, it must be fed continuously as a component of the feedstock, suitably at a concentration of several hundred parts per million.
Naphtha which is rendered free of arsenic can be used as other preferred feedstocks and products, for example:
(a) Feed to Pt catalytic reforming where As is a serious poison. PA1 (b) Gasoline blending. PA1 (c) Feed to noble metal catalyst pretreating. PA1 (d) Feed for C.sub.5 and C.sub.6 isomerization using Pt/Pd catalysts.
2. Description of the Prior Art
As stated, there is no known prior art relating to the selective removal of arsenic compounds from hydrocarbons in the presence of organic sulfur compounds. This applies to gas, gas-liquids (LPG) and liquid hydrocarbons, such as naphtha and light distillates.
U.S. Pat. Nos. 3,782,076, 3,789,581, 3,542,669 and 4,849,577 relate to arsenic removal in the absence of organic sulfur contamination.
Known catalysts or sorbents for removal of arsenic include PbO/Al.sub.2 O.sub.3, CuO/gammaAl.sub.2 O.sub.3 and CuO/ZnO/gammaAl.sub.2 O.sub.3. These materials remove or react with H.sub.2 S, COS, RSH (mercaptans), and AsH.sub.3.
Normally, other methods are used to remove H.sub.2 S and RSH whenever possible because such other methods are cheaper, thus leaving AsH3 and COS clean up for the sorbents listed above. All of these impurities would otherwise compete with each other for sorption by the arsenic sorbents.
U.S. Pat. Nos. 3,782,076 and 4,849,577 as well as an article Remove Arsine to Protect Catalyst, N. L. Carr, D. L. Stahlfeld and H. G. Robertson, Hydrocarbon Processing, May 1985, pages 100-102, all relate to processes for removal of arsenic from hydrocarbon streams. However, none of these background processes relate to a problem regarding mercaptan and non-mercaptan sulfur compounds in catalyst deactivation during the arsenic removal process.